The movie begins with Chase Sui Wonders as Ava Brooks, arriving at her childhood home in preparation for her best friend Madelyn Cline’s engagement party, which is set to take place during a lively Fourth of July weekend. Ava is still grappling with the emotional pain of losing her mother, and she feels a mixture of excitement and anxiety about seeing old friends and potential sparks with her high school ex-boyfriend, Jonah Hauer-King as Milo Griffin.
As the friends gather that night, reconnecting after a long time apart, the mood shifts between cheerful reminiscing and underlying tension. Danica Richards, played by Madelyn Cline, is now engaged to Teddy Spencer, portrayed by Tyriq Withers. Teddy is the son of local real estate mogul Grant, played by Billy Campbell, and his wife Jill, brought to life by Simone Annan. During the celebration, Milo still holds feelings for Ava, but he insists to Teddy that their relationship is just friendship.
The night takes an unexpected turn when the group encounters a former friend, Stevie Ward, played by Sarah Pidgeon, and her friend Hannah, portrayed by Georgia Flood. Stevie is bitter about being left behind and feeling abandoned after high school, but she accepts the invitation to join the group. The five of them then set off on a drive through the town, stopping to watch a spectacular fireworks display. Amidst the festivities, Teddy’s reckless behavior in the street nearly gets him hit by a car until Milo pushes him out of harm’s way, which raises some concern from the others.
Suddenly, a pickup truck swerves sharply to avoid hitting Teddy and crashes off the road into the rocks below. The group tries desperately to free the trapped driver and prevent the truck from slipping further, but despite their efforts, the vehicle falls. Milo calls the police, but the assailants quickly flee the scene, leaving the group shaken. Danica makes Ava promise not to reveal what really happened, fearing repercussions.
A year later, the story picks up at a bridal shower in Southport, again during the July Fourth festivities. Ava reunites with friends and meets a young woman named Tyler, played by Gabbriette Bechtel, who mentions her podcast "Live Laugh Slaughter" and expresses fascination with the notorious Southport massacre from 1997. During the party, news breaks that Danica and Teddy have split, and she is now engaged to Wyatt, portrayed by Joshua Orpin. It is also revealed that Stevie has been helping Danica over the past year, while Ava has not been in contact with her.
While opening gifts, Danica receives a disturbing card that reads "I KNOW WHAT YOU DID LAST SUMMER", which she shows only to Ava, Milo, and Stevie, suspecting it might be from Teddy, who still bears a grudge over their breakup. Curious and unsettled, the friends decide to investigate. They head to Teddy’s boat at the docks, where Teddy, now often drunk, dismisses their concerns and denies sending any threatening messages.
Later that night, Danica and Wyatt are alone at home. Danica takes a bath while Wyatt becomes suspicious of noises outside. When he looks out, he sees no one, but then is brutally attacked with a harpoon gun in his own backyard by someone dressed in a slicker, wielding a hook—the hallmark weapon of The Fisherman, the infamous killer from the original Southport massacre. The attacker slashes Wyatt with a hook and leaves him dead, with his body posed upright and the words "You can’t erase the past" written in his blood on the wall.
The friends assemble at the police station, trying to process Wyatt’s murder. As they reflect on the recent attacks, they realize that Wyatt was not with them during the incident and that his body was discovered far from their whereabouts. The police are unable to find concrete links to the original victim Sam Cooper, complicating the investigation. Ava, recalling Tyler’s obsession with the massacre, arranges to meet her in hopes of uncovering clues. Tyler explains the history of the killings and then takes Ava to the infamous Bayside House, where the original attack happened.
As they explore the house, the current Fisherman appears and gruesomely attacks them. Ava is trapped in a cage, and Tyler fights back upstairs. Stevie unexpectedly arrives after receiving a worried message from Ava. She manages to free Ava, but Tyler is murdered—hanged out of a second-story window, with the killer clearly recognizing her and muttering “You…” before killing her. Witnessing this horror, Ava and Stevie are left distraught and shaken.
In a town hall meeting, some locals, including Grant, try to dismiss the recent killings as a hoax or suicide—hoping to avoid scaring away tourists. Among the skeptics is Freddie Prinze Jr. as Ray Bronson, a survivor of the original massacre and Stevie’s boss. Instinctively anxious, Ava drives to a nearby town to seek out Jennifer Love Hewitt as Julie James, a woman who barely wants to revisit her traumatic past. Despite her reluctance, Julie recognizes Ava’s desperation and invites her to her home, cautioning her about the danger they face from the killer.
Ava, Milo, and Stevie then visit an impound lot, suspecting that Sam’s truck was taken there after the accident. Their investigation reveals that the vehicle belonged to a local pastor, Judah Gillespie, played by Austin Nichols, whose connection to the victim begins to unravel. They discover that Sam stole the truck from Judah, raising more questions about the past.
Meanwhile, Danica and Teddy visit the cemetery where they find graves of the original victims along with Sam Cooper's fresh grave, adorned with recently left flowers. They ask the groundskeeper to review security footage but are attacked when he is slashed by The Fisherman, who then pursues Danica into a building. She hides in a parade float but is eventually captured, though Teddy intervenes and fights the attacker off, leading him to bring Danica to safety.
The escalating violence reaches its peak when Ebby Yus as Man helps to reveal that Milo is also targeted. Tragically, Eric Fuzi as Fazzly, and others fall victim in the chaos. Milo is strangled and then killed with a snap of the neck by The Fisherman, who takes his phone to mislead Ava into thinking Milo has left for good.
Tensions come to a breaking point when Waheeda as Sahida and the remaining friends realize that Ray Bronson is not only aware of the killer but has become complicit. Ray visits Jennifer Love Hewitt once again, confronting her about his fractured past and his obsession with erasing the memories of the original massacre.
In their desperate attempt to survive, Danica and Ava go to the church, where they encounter Hannah. They start to connect the dots—that Judah’s resistance and the presence of Sam in old photographs link the past together. They attempt to go to the police, but Judah is found murdered, with his eyes gouged out, kneeling as if in an act of obsession. The shocking discovery then reveals that Stevie, once thought to be a victim, is actually connected to the original killer. She is seen in a shocking photograph standing next to Sam, suggesting she was involved all along.
The truth erupts in a dramatic climax at sea: Stevie drops anchor and reveals herself as The Fisherman. She confesses she was romantically involved with Sam after feeling betrayed by her friends, and she orchestrated the killings of her former friends’ loved ones, harboring a desire for revenge. The two women fight fiercely on the boat, with Danica and Ava trying to escape. Just as Stevie goes for a final, deadly attack with her hook, Ray arrives to intervene. After a tense struggle, Ray shoots Stevie into the water, rescuing Ava.
Despite their victory, the aftermath leaves more questions. Ava returns to Southport with Danica, and the two reflect on the tragedy as they sit by the beach, reuniting their friendship. Ava reveals that Stevie might still be out there, alive and dangerous. As the credits roll, a new figure, Karla Wilson, a survivor of the original massacre, watches the news with her husband. She then receives a sinister message from Stevie, now hinted to be alive and lurking—signaling that the nightmare may not be over and that the killer’s legacy could continue.
> “It’s not over."
